Environ. Scl. Technol. 1993, 27, 1381-1387
Characteristics and Environmental Significance of Colloids In Landfill Leachate Vaslllos Gounarls, Paul R. Anderson,. and Thomas
M. Holsen
Pritzker Department of Environmental Engineering, Illinois Institute of Technology, Chicago, Illinois 60616 Landfill leachate samples were collected from a municipal landfill and treated by microfiltration and ultrafiltration to separate their colloidal materials into specific size fractions. Each of these size fractions was then analyzed to quantify and characterize the colloidal materials and identify associated contaminants. The major component of the isolated colloids was organicmatter, which probably existed on the surface of the colloids and kept them in a stable suspension. Equilibrium calculations suggest that the inorganiccomponent of the colloids was predominantly carbonate and phosphate precipitates. Significant fractions of Zn, Pb, and Cr were associated with the colloids or complexed with dissolved organicligands. Hydrophobic organic pollutants were found to be strongly associated with colloids larger than 0.1 pm. Results of this study suggest that treatment processes for landfill leachate are likely to be affected by the associations between leachate pollutants and colloids. It is unlikely, however, that those associations can significantly enhance the movement of pollutants in contaminated aquifers. Introduction Pollutant movement in subsurface environments, and through treatment processes, may be enhanced by colloidfacilitated transport (1-4). Enhanced mobility and transport of hydrophobic pollutants due to their associations with humic material and sewage colloids have been demonstrated in laboratory experiments (ref 5, and references therein). Sheppard et al. (6) reported americium and lead moving 4 orders of magnitude faster than predicted in soil; they attributed this enhanced mobility to associations of these elements with mobile humic acids and colloids. In a well-studied site, macromolecules in secondarytreated sewage have been observed to cover fresh vivianite precipitate upon infiltration in the ground, forming mobile colloids (7).Those colloidstended to associatewith organic hydrophobic pollutants and potentially double the mobility of perylene and benzolalanthracene (8). Similar phenomena could take place in landfill leachate where (a) spatial variations in acidification (9)could favor dissolution and subsequent precipitation of metals along pH gradients, (b) high amounts of humic-like macromolecules exist (10) that may stabilize colloidal precipitates (111, and (c) a variety of organic and inorganic pollutants are present that could associate with colloidal and/or macromolecular species. The objectives of this study were to fractionate and characterize the colloids in landfill leachate, to identify the pollutants associatedwith the colloids in each fraction, and to assess the environmental significance of these associations. This investigation confirmed the existence of macromolecules and colloids in landfill leachate, revealed that significant fractions of organic and inorganic pollutants are associated with the colloids, provided data 0013-936X/93/0927-1381$04.00/0
0 1993 American Chemical Society
about the extent of these associations, and identified a certain subpopulation of colloids (0.1-1 pm) as the major potential carrier of organic hydrophobic pollutants in leachate-contaminated aquifers. Sampling, Fractionation, and Analyses
Sampling, Leachate samples were collected from a closed landfill that had received municipal refuse between 1970 and 1974. This site contains a network of passive landfill gas vents, many of which extend to the leachatesaturated zone. A previous confidential study indicated that a crack in the clay cover of the landfill was the major infiltration area. To study the impact of leachate evolution on colloidalsolids, most of the samples were collectedfrom two vents during selected seasons (Table I). One vent (DV-3) was near the infiltration area, and one vent (DV12) was used to sample the lowest measurable leachate elevation in the landfill. Leachate samples were withdrawn from the vents using a positive displacement pump driven by compressed nitrogen gas (Timco Isoomega). The pumping rate was kept very low (100 mL/min) to prevent detachment of colloidal material from the surrounding refuse (8). To ensure that collected samples were representative of bulk leachate, the conductivity, turbidity, dissolved oxygen (DO), and pH values of the pump effluent were continuously monitored during sampling. No sample was collected until all these index parameters had reached a steady value. The samples were preserved with sodium azide (1 g/L) and stored under nitrogen at a temperature slightly below the in situ temperature of the leachate. Fractionation. The leachate samples were then treated to separate and concentrate leachate colloids into size fractions. This fractionation was accomplished by membrane microfiltration and ultrafiltration in a stirred cell apparatus (Figure 1). The suspension to be concentrated was pumped under pressure through the stirring cell with the retentate being recycled back to the holding reservoir and the filtrate collected in a separate reservoir. As the volume of retentate decreased, the rejected colloids were concentrated. The final concentration ratios ranged from 8 to 15. The process was repeated for each size fraction. Each time the filtrate of the previous step was treated with membranes of smaller cutoff diameter until all the fractions were obtained. All leachate suspensions were stored in amber glass containers with air-tight, Teflon-lined caps. All pumps and tubing used in sampling and fractionation were made of Teflon. The walls of the stirred cell were equilibrated with the dissolved phase of the raw leachate to avoid sorptive losses during fractionation. Samples were collected, fractionated, and stored under a nitrogen atmosphere to prevent absorption of oxygen that could oxidize leachate constituents. The flow rate of the nitrogen gas stream was kept to a minimum to prevent excessive stripping of dissolved COZ. Total losses of Envlron. Sci. Technol., Vol. 27, No. 7, 1993
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induced X-ray emission (PIXE) analysis to identify elements that were associated with colloids. The PIXE sample location temp ( O C ) turbidity (NTU) date analyses were performed by the Element Analysis Corp. (Tallahassee,FL) using the residues of 0.1-0.2-mL aliquots 1 ventDV-13 24 18.1 Sep 22,1990 2 ventDV-3 18 10.5 Mar 8,1991 dried on thin Teflon membranes. Further analysis by 3 ventDV-12 30 550 Jun 8,1991 atomic absorption (Perkin Elmer 1lOOB AA spectropho4 ventDV-3 31 7.2 Sep 1,1991 tometer) quantified selected elements in subsequent 5 ventDV-12 34 58.0 Sep 2,1991 samples. Prior to analyses, these suspensions were acida Temperature and turbidity data represent steady-state conditions. ified (to pH < 1) and sonicated to redisperse precipitated humic acids. The amounts of total (TS) and fixed solids (FS) were measured by weighing the residue of 5 mL of suspension dried at 103 and 550 OC, respectively. TOC was obtained by taking the differencebetween total and inorganiccarbon measured by a total carbon analyzer (Dohrman DC-80). Bulk leachate from sample 1 was scanned for organic priority pollutants by GC and GC/MS according to EPA methods 608 (PCBs) and 624 (acid, base, and neutral extractables), respectively. Quantification of selected pollutants [PCBs and polycyclic aromatic hydrocarbons (PAHs)] in subsequent samples was performed with ( ~ U W ~ I T Y ' _ _ ~ _ _ capillary GC, and the identification of the PAHs was j z * / . > _. . u I confirmed with GUMS. Sample pretreatment consisted S3.EYOID ' d STIRREC of triple extraction with a 1:l mixture of hexane and methylene chloride (250-300 mL of leachate, 150 mL of Figure 1. Colloid fractionation apparatus. solvent each time), extract drying through a sodium sulfate column, concentration to 2 mL by distillation in a threeinorganic carbon (IC) were about 14% (raw leachate IC ball Snyder column (to10mL) and subsequent evaporation ranged from 772 to 1215mg/L), which could have increased under a N2 stream, cleanup with 10 g of 3 % deactivated the pH of the leachate by about 0.5 pH unit from its in silica gel in a cleanup column, elution with 200 mL of 6% situ value of 7.2. These changes in leachate chemistry ethyl ether solution in hexane, and concentration to 1mL correspond to a theoretical maximum increase of carbonate by the methods described above. ion concentration of 6.7 mg/L, which (if completely precipitated with major leachate metals) could produce Colloidal Concentration and Composition about 11 mg/L of solids. This amount is insignificant compared to the total amount of colloids found in the The first sample was used to verify the presence of leachate. Constant turbidity values of the leachate colloids and to test and optimize the sampling and solutions verified that no significant precipitate formation analytical protocols. The distribution of colloidal mass in took place during either sampling or fractionation. In leachate samples 2-5 is summarized in Figure 2 along with future studies, use of a gas mixture simulating landfill gas corresponding concentrations of dissolved solids. The could eliminate C02 loss. temporal and spatial variations in the concentration of Both the filtrate and retentate (fraction) of each submicron colloids were similar to the fluctuations in the filtration step were analyzed for total and fixed solids, concentration of dissolved solids (
